An aerosol is a combination of a gas and fine solid particles or liquid droplets suspended in the gas. It has been recognized that many environmental, medical, and industrial problems involve the chemistry of aerosols.
In the case of deleterious environmental effects, it is now known that the particulates suspended in the atmosphere can catalyze reactions in processes that depend both upon the size and the chemical composition of the particles. Modeling of atmospheric processes will require this complete characterization of a complex heterogeneous aerosol.
In the case of deleterious medical effects, many environments contain excessively high amounts of particles, such as coal dust or the like. Although many of the biochemical reactions have been established, the introduction of these particles into the body through the lungs is dependent upon the size of the particles.
In a real environment, the suspended particles are non-uniform, having a range of sizes and compositions. That is, the aerosol is polydisperse with particles. Furthermore, the particle concentration may be relatively low and not be amenable to many characterization techniques. Nonetheless, it is highly desirable to be able to measure both the size and chemical composition of particles in a polydisperse sample. It is also highly desirable that the characterization be performed in situ, allowing for an immediate determination of the aerosol's characteristics.
One of the present inventors in the parent application, incorporated herein by reference in its entirety, has disclosed a novel and useful analyzer capable of in situ analysis of both the size of particles and of their chemical composition. This technology has been reported also by: (1) Prather et al. in AReal-time characterization of individual aerosol particles using time-of-flight mass spectrometry,@ Analytical Chemistry, vol. 66, no. 9, 1994, pp. 1403-1407; and (2) Nordmeyer et al. in AReal-time measurement capabilities using aerosol time-of-flight mass spectrometry,@ Analytical Chemistry, vol. 66, no, 20, pp. 3540-3542. The analyzer includes two separate but interoperative parts. A first, sizing part 10, as illustrated in the schematic block diagram of FIG. 1, includes an aerosol generator 12, which in the case of environmental testing system may include only a tubular port to the gaseous environment being tested. For other applications, the aerosol generator 12 may include means for converting the substance being tested into an aerosol.
An aerosol interface 14 receiving the output of the aerosol generator 12 includes a nozzle and one or more skimmers between respective differentially pumped vacuum stages which convert the generally isotropic aerosol into a well defined beam of the aerosol environment with the velocity of the entrained particles being dependent upon the size of the those particles entrained within the aerosol. A preferred embodiment of the aerosol interface 14, as explained in the parent application, includes a nozzle through which the aerosol is supersonically accelerated with the particles entrained in the gas. The nozzle is conically shaped with a small aperture in its tip facing downstream. Because of the rapid acceleration, the resultant velocity of the entrained particles is inversely related to the size of those particles, that is, upon the aerodynamic size of the particles. The skimmers are also conically shaped but with their tips facing upstream. They act to remove most of the air or other gas from the particle beam, thereby allowing an open path between atmosphere and the very low vacuum of the mass spectrometer and to create a well defined beam.
This beam of particles is incident upon a dual-laser tracking system 16 which includes two laser particle detectors arranged along the path of the accelerated particles. Each particle in passing the two laser detectors produces a precisely defined electrical pulse. A timing circuit 18 determines the temporal difference between the two pulses arising from the two axially arranged detectors. The temporal difference is proportional to the particle's velocity, which in turn is inversely related to the particle's size.
The timing circuit 18 also determines the temporal position of the particle along the beam path. The combination of the velocity and the temporal position allows the timing circuit 18 to determine when the particle passes an irradiation position within a time-of-flight mass spectrometer (TOFMS) 20. At that point, a dissociation laser 22 produces an intense light pulse directed at that point. The dissociation laser 22 ablates molecularly sized sub-particles from the already measured particle. Either the dissociation laser or a secondary laser ionizes the sub-particles into ionized sub-particles, for which the mass spectrometer 20 then analyzes the m/z ratio, that is, the ratio of the mass to the electronic charge of that particle. Assuming that the sub-particles are molecules, the m/z ratio provides in most cases a definitive determination of the chemical composition of the particle. Both the m/z ratio and the particle size are separately provided to a data acquisition unit 24 that can correlate the two sets of inputs. That is, the data acquisition unit 24 provides a spectrum of particles delineated both as to size and to m/z ratio of constituent atomically sized molecules which the laser irradiation desorbs from the macro particles.
The mass spectrometer described in the above patent application is in fact a unipolar spectrometer that allows the determination of either the positively charged state or the negatively charged state of the excited entrained atomically sized particles liberated by the dissociation laser. However, in a realistic polydisperse environment, both positively and negatively charged constituent molecules are present. Bipolar or dual ion spectrometers have been disclosed by Downey in U.S. Pat. No. 5,382,794.
The apparatus described above by Prather is in fact an experimental model that has been tested primarily in the laboratory. Its usefulness in a field environment has not been established. A field-usable instrument should be rugged, should not be prone to disalignment during transport, should be relatively compact and movable, should have readily available power, and should not consume excessive power.